116 research outputs found
A comprehensive chemical abundance study of the outer halo globular cluster M 75
Context: M 75 is a relatively young Globular Cluster (GC) found at 15 kpc
from the Galactic centre at the transition region between the inner and outer
Milky Way halos. Aims: Our aims are to perform a comprehensive abundance study
of a variety of chemical elements in this GC such as to investigate its
chemical enrichment history in terms of early star formation, and to search for
any multiple populations. Methods: We have obtained high resolution
spectroscopy with the MIKE instrument at the Magellan telescope for 16 red
giant stars. Their membership within the GC is confirmed from radial velocity
measurements. Our chemical abundance analysis is performed via equivalent width
measurements and spectral synthesis, assuming local thermodynamic equilibrium
(LTE). Results: We present the first comprehensive abundance study of M 75 to
date. The cluster is metal-rich ([Fe/H]=-1.16+/-0.02 dex,
[alpha/Fe]=+0.30+/-0.02 dex), and shows a marginal spread in [Fe/H] of 0.07
dex, typical of most GCs of similar luminosity. A moderately extended O-Na
anticorrelation is clearly visible, likely showing three generations of stars,
formed on a short timescale. Additionally the two most Na-rich stars are also
Ba-enhanced by 0.4 and 0.6 dex, respectively, indicative of pollution by lower
mass (M ~ 4-5 M_Sun) Asymptotic Giant Branch (AGB) stars. The overall n-capture
element pattern is compatible with predominant r-process enrichment, which is
rarely the case in GCs of such a high metallicity.Comment: 15 pages, 11 figures, 7 tables, accepted for publication in A&
The Galactic evolution of sulphur as traced by globular clusters
Sulphur is an important, volatile alpha element but its role in the Galactic
chemical evolution is still uncertain. We derive the S abundances in RGB stars
in three Galactic globular clusters (GC) that cover a wide metallicity range
(-2.3<[Fe/H]<-1.2), namely M4, M22, and M30. The halo field stars show a large
scatter in the [S/Fe] ratio in this metallicity span, which is inconsistent
with canonical chemical evolution models. To date, very few measurements of
[S/Fe] exist for stars in GCs, which are good tracers of the chemical
enrichment of their environment. However, some light and alpha elements show
star-to-star variations within individual GCs and it is yet unclear whether
sulphur also varies between GC stars. We used the the infrared spectrograph
CRIRES to obtain high-resolution (R~50000), high signal-to-noise (SNR~200 per
px) spectra in the region of the S I multiplet 3 at 1045 nm for 15 GC stars
selected from the literature (6 stars in M4, 6 stars in M22 and 3 stars in
M30). Multiplet 3 is better suited for S abundance derivation than the more
commonly used lines of multiplet 1 at 920 nm, since its lines are not blended
by telluric absorption or other stellar features at low metallicity. We used
spectral synthesis to derive the [S/Fe] ratio of the stars assuming local
thermodynamic equilibrium (LTE). We find mean [S/Fe] = 0.58 +/- 0.01 +/- 0.20
dex (statistical and systematic error) for M4, [S/Fe] = 0.57+/-0.01+/-0.19 dex
for M22, and [S/Fe] = 0.55+/-0.02+/-0.16 dex for M30. The negative NLTE
corrections are estimated to be in the order of the systematic uncertainties.
With the tentative exception of two stars with measured high S abundances, we
conclude that sulphur behaves like a typical alpha element in the studied
Galactic GCs, showing enhanced abundances with respect to the solar value at
metallicities below [Fe/H] = -1.0 dex without a considerable spread.Comment: 9 pages, 7 figures, accepted for publication in A&
Precise abundance analysis of the outer halo globular cluster M 75
Globular clusters (GCs) are the oldest stellar systems in the Milky Way. Long
time considered as simple stellar populations, nowadays we recognize their
complex star formation history through precise abundance analysis of a variety
of chemical elements in individual cluster members. Although we do not
necessarily see clues for multiple populations in all GC colour-magnitude
diagrams, all GCs present significant spreads and certain anticorrelations
between their light and alpha element abundances. Furthermore, the heavy
element abundances in individual stars of the primordial generation and their
comparison to halo field stars and dwarf galaxies could provide us with
valuable information about the very first stars that could have formed in GCs.
M75 is a unique outer halo (galactocentric distance of ~15 kpc) GC with a
peculiar Horizontal Branch morphology. Here we present the first abundance
measurements of 16 individual red giants from high resolution spectroscopy. The
cluster is metal rich ([Fe/H] = -1.17 +/- 0.02), alpha-enhanced, and shows a
marginal spread in [Fe/H] of 0.07 dex, typical of most GCs of similar
luminosity. The O-Na anticorrelation is clearly visible, showing at least two
generations of stars, formed on a short timescale. We also discuss r- and
s-process element abundances in the context of the earliest cluster enrichment
phases.Comment: 3 pages, FIRST STARS IV conference proceedin
Kinematics of outer halo globular clusters: M 75 and NGC 6426
Globular clusters (GCs) and their dynamic interactions with the Galactic
components provide an important insight into the structure and formation of the
early Milky Way. Here, we present a kinematic study of two outer halo GCs based
on a combination of VLT/FORS2, VLT/FLAMES, and Magellan/MIKE low- and
high-resolution spectroscopy of 32 and 27 member stars, respectively. Although
both clusters are located at Galactocentric distances of 15 kpc, they have
otherwise very different properties. M 75 is a luminous and metal-rich system
at [Fe/H] = dex, a value that we confirm from the calcium triplet
region. This GC shows mild evidence for rotation with an amplitude of A5 km s. One of the most metal-poor GCs in the Milky Way (at
[FeII/H] = dex), NGC 6426 exhibits marginal evidence of internal
rotation at the 2 km s level. Both objects have velocity dispersions
that are consistent with their luminosity. Although limited by small-number
statistics, the resulting limits on their ratios suggest
that M 75 is a slow rotator driven by internal dynamics rather than being
effected by the weak Galactic tides at its large distances. Here, M 75 () is fully consistent with the properties of other, younger
halo clusters. At , NGC 6426 appears to have a
remarkably ordered internal motion for its low metallicity, but the large
uncertainty does not allow for an unambiguous categorization as a fast rotator.
An accretion origin of M 75 cannot be excluded, based on the eccentric orbit,
which we derived from the recent data release 2 of Gaia, and considering its
younger age.Comment: 9 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
Chemical abundances and kinematic properties of Galactic globular clusters
High-resolution spectroscopic studies of globular clusters (GC) have been a major break-through
in our understanding of these ancient stellar systems. The observations carried out in the last decade
revealed a very heterogeneous population of objects – not only have GCs undergone a complex
star formation history resulting in large chemical inhomogeneities common amongst all of them,
but there are also non-negligible cluster-to-cluster differences in both chemistry and kinematics that
are not yet fully understood. In this thesis, we present the first ever high-resolution studies of two
poorly known GCs M75 and NGC4372. M75 is a massive, relatively metal-rich, outer halo GC
with extremely broad horizontal branch. We found that, besides the typical light-element variations,
it shows a marginal spread in metallicity and likely has a small s-process rich population. Surprisingly
for its metallicity, the majority of the analysed stars lack s-process enrichment, which hints
to a formation in an environment that built metals very quickly. On the other hand, NGC4372 is
one of the most metal-poor GCs in the Galaxy. It is found in the inner halo and has experienced
multiple disk crossings. The chemical analysis revealed it as a standard representative of the old,
metal-poor halo group. More interesting are its structural and kinematic properties as the cluster
has an unusually high intrinsic rotation for its metallicity and appears to be rotationally flattened.
Furthermore, since GCs trace the chemical properties of their environment, the thesis also includes
the first homogeneous study of the Galactic halo evolution of the poorly studied a element sulphur
traced by GC stars covering a large metallicity span
A study of rotating globular clusters - the case of the old, metal-poor globular cluster NGC 4372
Aims: We present the first in-depth study of the kinematic properties and
derive the structural parameters of NGC 4372 based on the fit of a Plummer
profile and a rotating, physical model. We explore the link between internal
rotation to different cluster properties and together with similar studies of
more GCs, we put these in the context of globular cluster formation and
evolution. Methods: We present radial velocities for 131 cluster member stars
measured from high-resolution FLAMES/GIRAFFE observations. Their membership to
the GC is additionally confirmed from precise metallicity estimates. Using this
kinematic data set we build a velocity dispersion profile and a systemic
rotation curve. Additionally, we obtain an elliptical number density profile of
NGC 4372 based on optical images using a MCMC fitting algorithm. From this we
derive the cluster's half-light radius and ellipticity as r_h=3.4'+/-0.04' and
e=0.08+/-0.01. Finally, we give a physical interpretation of the observed
morphological and kinematic properties of this GC by fitting an axisymmetric,
differentially rotating, dynamical model. Results: Our results show that NGC
4372 has an unusually high ratio of rotation amplitude to velocity dispersion
(1.2 vs. 4.5 km/s) for its metallicity. This, however, puts it in line with two
other exceptional, very metal-poor GCs - M 15 and NGC 4590. We also find a mild
flattening of NGC 4372 in the direction of its rotation. Given its old age,
this suggests that the flattening is indeed caused by the systemic rotation
rather than tidal interactions with the Galaxy. Additionally, we estimate the
dynamical mass of the GC M_dyn=2.0+/-0.5 x 10^5 M_Sun based on the dynamical
model, which constrains the mass-to-light ratio of NGC 4372 between 1.4 and 2.3
M_Sun/L_Sun, representative of an old, purely stellar population.Comment: Accepted for publication in A&A, 12 pages, 14 figures, 2 table
Strömgren uvby photometry of the peculiar globular cluster NGC 2419
NGC 2419 is a peculiar Galactic globular cluster offset from the others in the size-luminosity diagram, and showing several chemical abundance anomalies. Here, we present Strömgren uvby photometry of the cluster. Using the gravity- and metallicity-sensitive c1 and m1 indices, we identify a sample of likely cluster members extending well beyond the formal tidal radius. The estimated contamination by cluster non-members is only one per cent, making our catalogue ideally suited for spectroscopic follow-up. We derive photometric [Fe/H] of red giants, and depending on which metallicity calibration from the literature we use, we find reasonable to excellent agreement with spectroscopic [Fe/H], both for the cluster mean metallicity and for individual stars. We demonstrate explicitly that the photometric uncertainties are not Gaussian and this must be accounted for in any analysis of the metallicity distribution function. Using a realistic, non-Gaussian model for the photometric uncertainties, we find a formal internal [Fe/H] spread of σ=0.11+0.02-0.01 dex. This is an upper limit to the cluster’s true [Fe/H] spread and may partially, and possibly entirely, reflect the limited precision of the photometric metallicity estimation and systematic effects. The lack of correlation between spectroscopic and photometric [Fe/H] of individual stars is further evidence against a [Fe/H] spread on the 0.1 dex level. Finally, the CN-sensitive δ4, among other colour indices, anti-correlates strongly with magnesium abundance, indicating that the second-generation stars are nitrogen enriched. The absence of similar correlations in some other CN-sensitive indices supports the second generation being enriched in He, which in these indices approximately compensates the shift due to CN. Compared to a single continuous distribution with finite dispersion, the observed δ4 distribution of red giants is slightly better fit by two distinct populations with no internal spread, with the nitrogen-enhanced second generation accounting for 53 ± 5 per cent of stars. Despite its known peculiarities, NGC 2419 appears to be very similar to other metal-poor Galactic globular clusters with a similarly nitrogen-enhanced second generation and little or no variation in [Fe/H], which sets it apart from other suspected accreted nuclei such as ωCen
Stellar populations and star formation histories of the nuclear star clusters in six nearby galaxies
The majority of spiral and elliptical galaxies in the Universe host very
dense and compact stellar systems at their centres known as nuclear star
clusters (NSCs). In this work we study the stellar populations and star
formation histories (SFH) of the NSCs of six nearby galaxies with stellar
masses ranging between and (four late-type
spirals and two early-types) with high resolution spectroscopy. Our
observations are taken with the X-Shooter spectrograph at the VLT. We make use
of an empirical simple stellar population (SSP) model grid to fit composite
stellar populations to the data and recover the SFHs of the nuclei. We find
that the nuclei of all late-type galaxies experienced a prolonged SFH, while
the NSCs of the two early-types are consistent with SSPs. The NSCs in the
late-type galaxies sample appear to have formed a significant fraction of their
stellar mass already more than Gyr ago, while the NSCs in the two
early-type galaxies are surprisingly younger. Stars younger than Myr are
present in at least two nuclei: NGC 247 and NGC 7793, with some evidence for
young star formation in NGC 300's NSC. The NSCs of the spirals NGC 247 and NGC
300 are consistent with prolonged \in situ star formation with a gradual
metallicity enrichment from dex more than Gyr ago, reaching
super-Solar values few hundred Myr ago. NGC 3621 appears to be very metal rich
already in the early Universe and NGC 7793 presents us with a very complex SFH,
likely dominated by merging of various massive star clusters coming from
different environments.Comment: Accepted for publication in MNRA
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